As heat-sensitive recording material, there has been known a material having a heat-sensitive layer on a substrate such as paper. In the heat sensitive layer, a color former and a color developer which produces color when it contacts with the color former, and a colored image can be obtained by heating with, what is called, a heating pen or thermal head.
Such a heat-sensitive recording material is relatively cheap and is used as a recording medium in various fields such as facsimile, various calculators, medical instruments, computers, heat-sensitive copying machines and printers of other various instruments and apparatus.
However, since development of various office machines and diversification of their use progress rapidly, it is desirable to develop a new heat-sensitive recording material which can meet respective requirements. For example, in order to meet requirements for making the operating speed of recording apparatuses fast, a recording material which can provide a clear image having a high density even with very low printing energy is desired. For this purpose, it has been recognized that not only a heat-sensitive layer itself but also its substrate should be studied and, thereby, use of synthetic paper or synthetic resin film instead of conventional natural paper have been increased.
As means to cope with low printing energy due to high-speed printing, for example, a heat-sensitive recording material disclosed in Japanese Patent Kokai No. 59-171685 has an undercoat layer containing minute cells and having excellent elasticity and heat insulating properties which is formed by providing a layer composed of a thermal expanding agent and a thermoplastic polymer on a substrate and heating the layer. In this heat-sensitive recording material, a clear recorded image having a relatively high density can be obtained even with low printing energy because of formation of the undercoat layer having elasticity and heat insulating properties. However, for producing the recording material, a step for expanding the thermal expanding agent is required. In addition, it is very difficult to control the degree of expansion in this step, and a uniformly expanded layer can hardly obtained. Therefore, there is a defect in reproducibility of fine and thin images such as that required in, for example, a video printer. On the other hand, Japanese Patent Kokai No. 59-225987 discloses a method for improving evenness by providing a pigment coating layer on an expanded layer. However, since minute uneveness still remains on the recording layer, it is difficult to obtain a heat-sensitive recording material having sufficient resolution.